The broad objective of the proposed experiments is to identify genes whose function is important in cardiac development. The project takes as a starting point tinman, a homeodomain protein that is essential to the development of the embryonic heart and visceral muscle of Drosophila. Closely related genes are active in vertebrate heart development. The existence of a conserved regulator of cardiogenesis in flies, fish frogs, chickens and mice strongly argues that the heart evolved once in a common ancestor of an of these species. The proposed experiments will exploit this commonalty to study cardiogenesis in the powerful model system of Drosophila with the aim of furthering our understanding of cardiogenesis in vertebrates. The hypothesis to be tested is that tinman regulates genes that are required for cardiogenesis and cardiac function in insects and vertebrates. Specific Aims: 1) To identify fragments of genomic Drosophila DNA that are recognized by the homeodomain of tinman by genetic selection in yeast. 2) To identify transcription units near the isolated fragments and analyze the pattern of their expression in wild type and tinman mutant embryos to find genes that are regulated by tinman. 3) To map the proposed tinman binding sites to polytene chromosomes in order to identify known candidate genes for tinman regulation. 4) To determine if the genomic fragments recognized by the homeodomain of tinman in yeast can confer tinman regulation on a reporter gene in Drosophila and mice. 5) To determine the requirement in cardiogenesis for both novel and known candidate genes by analyzing known mutations or by obtaining new mutations in these genes. 6) To identify vertebrate homologues for novel genes, and to determine if the vertebrate homologues of the tinman target genes are targets of vertebrate tinman homologues. 7) To employ a versatile system for regulated ectopic expression of tinman to determine its ability to function as a master regulator of cardiogenesis. 8) To perform a saturating mutagenesis screen for genes required for cardiogenesis in Drosophila by employing green fluorescent protein to visualize the embryonic heart. This research is most likely to find applications in the clinical contexts of congenital heart disease, dilated cardiomyopathies both inherited and acquired, and cardiac hypertrophy. The mechanisms governing cardiac development are likely to play a role in both inadequate and excessive growth of myocardium in both the child and the adult.